Thulium oxide heat source and method for forming same

ABSTRACT

An internally shielded radioisotopic heat block with the preferred fuel, thulium oxide rods, separated from the coolant by a metal heat-exchange medium. The fuel elements may be removed from the heat block with only moderate shielding and reirradiated while still clad.

United States Patent Inventors Charles R. Allen;

Charles E. Leach, both of Paseo, Wash. 751,454 Aug. 9, 1968 Sept. 7, 1971 The United States of America as represented by the United States Atomic Energy Column A ppl No Filed Patented Assignee [56] References Cited UNITED STATES PATENTS 3,306,045 2/1967 BufordJr. 61:11. 176/39 3,350,231 10/1967 HelllZ 176/10 3,353,354 11/1967 Friedmanetal. 176/39 3,373,449 4/1968 RObel'tSeIaL... 176/39 3,378,455 4/l968 1x61116131. 176/89 3,421,001 1/1969 Fitzgeraldetal. l76/l0 OTHER REFERENCES S. Untermger, May 1954-Nucleonles (Vol. 12 No. 5) pp. 35,36.

Primary Examiner-Reuben Epstein AttorneyRoland A. Anderson 1 ABSTRACT: An internally shielded radioisotopic heat block with the preferred fuel, thulium oxide rods, separated from the coolant by a metal heat-exchange medium. The fuel elements may be removed from the heat block with only moderate shielding and reirradiated while still clad.

TIIULIUM OXIDE HEAT SOURCE AND METHOD FOR FORMING SAME CONTRACTUAL ORIGIN OF THE INVENTION The invention described herein was made in the course of, or under, a contract with the UNITED STATES ATOMIC ENERGY COMMISSION.

BACKGROUND OF THE INVENTION This invention relates to a radioisotopic heat source and more particularly to an internally shielded heat source with fuel rods of thulium oxide.

Use of radioisotopes as a heat source is not new, but the proximity of extended manned space and undersea missions has spurred development of a reliable, portable, easily serviced, isotopic heat source. Some isotopes available for and applicable as heat sources are Ru, Co, "Sr, Ce, "Cs and ""Tm. Each has characteristics which make it use advantageous depending upon the required design parameters. Watts per gram, density, power density, availability, type of irradiation, amount of required shielding, half life and cost are important characteristics.

Tm upon absorption of a neutron becomes ""Tm which is a radioisotope that decays to or Tm O has superior characteristics to the metal for many purposes and is the preferred form. "Tm O with a half life of about four months, a density of 8.5 gms/cm, a power density of 1.2 watts per gram of 90 weight percent "Tm.,,0;,l weight percent Tm- 0 and with only moderate shielding requirements seems .a good .compromise for manned missions of about 3 to 6 months. Thulium has a very large thermal-neutron cross section, hence a very small thickness of thulium absorbs large numbers of neutrons. This is significant because of the neutron flux depression caused by the presence of thulium in a reactor and also because in a thick piece of thulium not many neutrons reach the inner portion of the piece to produce the radioisotope Belgian Pat. No. 685,023 issued to Cambridge Nuclear Feb. 3, 1967) discloses irradiation of thin TM O wafers which were assembled into blocks and used as a heat source. As explained below neither wafers nor an assembly of wafers is acceptable as a power source for the above mentioned application.

SUMMARY OF THE INVENTION The invention comprises the irradiation of Im O in the form of encapsulated rod in a properly balanced thermalepithermal flux .to produce a'suitable Im O fuel element for use as a heat source in which the fuel elements are separated .from the coolant by a heat-exchange medium that also acts as an internal radiation shield. A backup coolant system separate from the primary coolant system ensures overall system integrity.

Since fuel elements have to be encapsulated in a cladding material which is usually welded, the rod shaped fuel element is more reliable than an assembly of wafers because only two welds are required per rod whereas welds are required for each wafer in the assembly. If the assembly contains 10 to wafers there is 10 to 20 times greater a chance of a bad weld with the wafers than with the rod. One bad weld may result in an entire heat source becoming inoperable or in .an entire piece of equipment becoming contaminated. Also, rods are not as delicate as wafers and not as likely to be damaged by handling. For a system which is to serve manned space or undersea missions removed from sophisticated repair facilities, a

tento twenty-fold greater chance of failure would probably prevent the use of the system altogether. Because of the short half life of Wm, fuel replacement is important.

An assembly of wafers prevents easy relocation or replacement of fuel elements because of the required shielding. As the lm O is more concentrated in the wafer assembly than in rods, the radiation field is more intense, and portable shields which may be used by a person with the proper gloves,

etc., to remove or relocate fuel rods, must be replaced by remotely controlled equipment with the concomitant problems and additional weight. It is clear that realignment or replacement of a wafer assembly is not practical in a space or undersea mission while realignment or replacement of rod is relatively simple After the rods or wafers have been removed they can be reirradiated to convert more of the l"m O to 'Tm O Reirradiation cost is another area in which rods provide advantages over wafers. Rods may be reirradiated as complete fuel elements exactly as they are removed from the heat source because their geometry as fuel elements is the same with respect to Tm O thickness as when they were initially irradiated. In contrast, wafers must be disassembled from their fuel element and reirradiated as wafers then reassembled before they are reusable as fuel elements. The necessary time, labor and shielding requirements make wafer reirradiation expensive compared to rod reirradiation.

All these reasons for the use of rods instead of wafers did not result in the production of rods because it was thought by experts that depending upon the rod density neutron irradia tion of Tm O rods would result in activation of the skin or outermost parts of the rod but not the core. It was believed that activation to about 20 mils was the maximum for material about percent theoretical density and to that end wafers greater than about 40 mils thick were not produced. Uniform activation of thulium fuel wafers or rods is required for acceptable fuel cost and power density. Because an equilibrium is reached during the irradiation of Wm at which the rate of ""Tm production equals the rate of "Tm decay, there is a limit to the amount of Tm which can be produced from a given Tm source in a given neutron flux. If the given *Im available for conversion to 'Im is less than if the given source is the entire rod. Uniform activation results from use of the entire rod as a source, hence more ""Tm production, no wasted "Im, lower fuel cost and higher power density.

While it is true that usual techniques for irradiation of Tm O rods will produce "Trn O only in the rod skin it has now been discovered that irradiation with correct proportions of thermal and epithermal neutrons will produce substantially uniform activation across significantly thicker area than heretofore possible. Epithermal neutrons have an energy level greater than 0.683 electron volts and less than 60,000 electron volts while thermal neutrons have an energy level less than 0.683 electron volts. Since it has been found that thulium has a smaller neutron cross section for epithermal neutrons than for thermal neutrons, at agiven neutron flux more epithermal neutrons will penetrate a specific thickness of thulium before being absorbed than thermal neutrons. Since it is primarily the thickness of the thulium which affects the activation distribu' tion and the neutron flux, rods of varied lengths may be irradiated. The availability of fm O rods has resulted in the novel heat source which constitutes this invention.

BRIEF DESCRIPTION OF THE DRAWiNGS The invention may be better understood by reference to the following drawings in which:

FIG. 1 is a partial section view of the heat source of this invention.

FIG. 2 is an enlarged partial section view of a fuel element.

FIG. 3 is a section view taken on line 3--3 in FIG. 1. FIG. 4 is a section view taken on line 4-4 in FIG. 1.

FIG. 5 is a graph showing the decrease in average activation for rods of increasing diameter in in a flux of thermal and epithermal neutrons.

FIG. 6 is a graph of the relationship between activation and depth in a Tm O, rod for different flux spectra.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT As shown in FIG. I, a heat block 10 consists of fuel elements 11 arranged in clusters manifolds surrounded by a heat exchange medium 13. The fuel elements 11 and heatcxchange medium 13 are housed within a tubular body 14 closed at the bottom end between an upper head 15 and a lower head 16. Coolant tubes 17 are U-shaped and mate at both ends with manifolds 18 in lower head 16. Intermediate their ends, the coolant tubes 17 follow an upward path through the heat-exchange medium 13 proximate the fuel element clusters 12 to a point near the upper head 15 where they become parallel to the upper head for a distance and turn downward to the manifolds 18.

Upper head 15 has an annular channel 19 in its lower edge which houses an inlet header 20. Backup coolant tubes 21 are connected to inlet head 20 and to an outlet header 22 located at the bottom of the tubular body 14. The paths of backup coolant tubes 21 between the inlet header 20 and the outlet header 22 closely approximate and are predominantly parallel to the paths of coolant tubes 17. Backup coolant enters inlet header 20 through backup coolant inlet 23 via a valve 24 located outside heat block 10 intermediate inlet header 20 and backup coolant inlet 23. In parallel arrangement to valve 24 and in thermal contact with body 14 or heat-exchange medium 13 is a fusible plug 25.

Upper head 15 and lower head 16 are connected to flanges 26 of body 14 by a plurality of screws 27 around the body circumference and by a nut 28 and bolt 29 assembly extending through the center of the body. Tubular insulating material 30 surrounds bolt 29.

With reference to FIG. 4, each manifold 18 in lower head 16 consists of an aperture 31 through the lower head and three grooves 32 connected to the aperture. Each groove 32 receives a coolant tube 17, and each aperture 31 leads to one of a series of collector rings 33. As shown in FIGS. 1 and 3, each fuel element cluster 12 rests on plate 34 which is attached to the bottom of body 14; the plates 34 are elliptical and each receives 10 fuel elements 11. FIG. 3 shows the spa tial relationship between fuel element clusters 12, coolant tubes 17 and backup coolant tubes 21. Fuel elements 11 extend through upper head 15 to plugs 35 which are easily removed from the upper head by means of bolts 36.

With reference to FIG. 2, fuel elements 11 are housed within fuel tubes 37 and rest on plates 34. Each fuel element 11 has an encapsulated fuel rod 38 and a smaller diameter shaft 39 connected to the top of the fuel rod. Shaft 39 has a slot 40 and fits inside and is slidable with respect to a sheath 41 which is the same diameter as the fuel rod 38. Sheath 41 has an internal roll pin 42 which fits inside of and is slidable with slot 40. Roll pin 42 connects sheath 41 to shaft 39 and fuel rod 38. A gripping portion or knob 43 is formed at the top of sheath 41 and abuts plug 35 when fuel element 11 is in operating position. A spring 44 between fuel rod 38 and sheath 41 urges the sheath away from the fuel rod.

Heat block 10 may be assembled as follows, but the order of the steps is not critical and may be varied. Either "fm O pellets are encapsulated in a cladding material or Tm O powder is vibratorily compacted in a tube of cladding material to form each fuel rod 38 of fuel element 11. The cladding material may be selected from any of a number of artrecog nized materials, such as titanium, beryllium or zirconium, de pending upon factors like operating temperature and the amount of shielding required. Titanium is preferred over zirconium where less shielding is desired because zirconium becomes more radioactive than the titanium. Fuel rods 38 are inserted into a nuclear reactor in an area of suitably balanced thermal-epithermal flux. The amount of time fuel rods 38 are irradiated depend upon the flux field, the rod diameter and density as well as the desired activation level. Elongated rods 0.171 inches in diameter and up to 1.75 inches in length have been substantially uniformly irradiated.

Fuel tubes 37 are placed within body 14 at their proper places on plates 34 in the desired cluster configuration 12. The exact geometry of clusters 12 is not critical, it depends upon the number of fuel elements 11 to be used and the number of coolant tubes 17. After clusters 12 are positioned,

the coolant tubes 17, backup coolant tubes 21 and outlet header 22 are added within the body 14. The heat-exchange medium 13 can be selected from a variety of materials depending upon operating temperature, required heat transfer coefficients, component compatibility and shielding requirements. Various metals in solid, liquid or powdered form are applicable as the heat-exchange medium 13 with powdered tungsten the preferred material. Tungsten is sufficiently dense to provide an adequate internal shield for manned missions while at the same time it has good heat transfer and compatibility characteristics. Mo, Na, Hg or Cu are alternatives provided the accompanying increased radiation intensity and/or shielding requirements are acceptable. The heat-exchange medium 13 of powdered tungsten is vibratorily compacted in body 14 to form a solid mass around the aforementioned components.

The inlet header 20 is connected to backup coolant tubes 21 then upper head 15 and lower head 16 are added to complete heat block 10 except for fuel elements 11. An external backup coolant supply (not shown) is connected to backup coolant inlet 23 and may consists of water under pressure in any kind of convenient container.

After irradiation, under conditions hereinafter explained, to produce fuel rods 38 of about l0 to about 20 weight percent ""Tm o substantially uniformly scattered throughout the remaining Tm O slotted shafts 39, sheaths 41 and springs 44 are added to complete fuel elements 11. Fuel elements 11 are loaded into fuel tubes 37 and pressure is applied at the top of the sheaths 41 to compress springs 44. Plugs 35 are added which maintain the fuel elements 11 in the fuel tubes 37. With addition of fuel elements 11, the heat block 10 is complete and a coolant such as water, helium or other suitable material is supplied from a source (not shown) to coolant tubes 17 via collector ring 33. It should be noted that some type of insulating material (not shown) will surround heat block 10.

In operation, decay of the Tm O in fuel elements 11 produces heat which is transmitted from the fuel elements through the fuel tubes 37 and heat-exchange medium 13 to coolant flowing through the coolant tubes 17. Coolant flows through tubes 17 to one of the collector rings 33 and thence to a heat-exchanger (not shown) where heat is extracted from the coolant. The coolant is returned to another of the collector rings and then into the coolant tubes for recirculation. If heat block 10 is used in combination with a power source based on the Stirling Cycle, then a gas coolant is preferred.

When a fuel element 11 is to be relocated or replaced, the mating plug 35 is removed. Spring 44 forces sheath 41 through upper head 15 until roll pin 42 contacts the end of slot 40. Movement of sheath 41 terminates at this point but knob 43 at the end of the sheath has been projected through the upper head 15 where it is easily accessible.

If for one reason or another the backup coolant system is required, there are two methods of activation. Valve 24 exter' nal to heat block 10 provides a nonemergency means for activation of the system; for instance, during startup, or shutdown or leak testing, etc. Fusible plug 25 in thermal contact with body 14 or heat-exchange medium 13 provides for cooling if extra programmed temperatures occur. Choice of the material for fusible plug 25 depends, of course, on the programmed temperature and the leeway provided for temperature excursions. The parallel arrangement of valve 24 and plug 25 enable the use of the backup coolant without destruction of the plug.

As previously stated use of thermal neutrons alone will not result in acceptable activation in a thulium oxide wafer greater than about 40 mils thick. Therefore, for rods greater than 0.04 inches in diameter flux fields of appropriately balanced ther mal and epithermal neutrons must be used. Flg. 5 shows the relationship between average neutron activation and radius for thulium oxide rods irradiated in a flux field comprised of 1 l0 neutrons/cm sec thermal neutrons and 5.6 10" neutrons/cm sec epithermal neutrons. A fuel rod 0.17l inches in diameter corresponds to a 0.218 cm radius and as shown in the Figure results in only slightly more than a 2 percent loss in average activation. For a rod 0.55 inches in diameter which corresponds to 0.698 cm radius, a 12.5 percent loss in average activation occurs. The greater the activation the more heat produced, so it is clear that the heat transfer characteristics of the heat block determine the permissible activation level of the fuel rods. Fuel rods 0.171 inches in diame ter have been prepared according to this invention which have all the benefits of rods over wafers yet lose only slightly more than 2 percent average activation. Rods up to about 0.5 inches in diameter may be used in heat blocks depending upon the particular design characteristics of each block.

FIG. 6 shows the relationship between activation in a Tm O rod as a function of neutron penetration depth and the neutron energy levels. The normalized activation is the ratio of localized, or incremental, activation to total activation. The ratio of activation at the rod skin, a theoretical depth of zero, to total activation is set equal to one. If the activation at any point in the rod was equal to that at the skin, then the normalized activation would always be one, but as previously explained, the large thermal neutron cross section prevents absolute uniform activation. For a flux field comprised of 5.6X10' neutrons/cm sec epithermal neutrons and 1X10 neutrons/cm sec thermal neutrons, curve A shows that the fraction of total rod activation due to epithermal neutrons is essentially constant from skin to core. Curve B shows that the fraction of total rod activation due to thermal neutrons decreases with increasing TM O thickness. Curve D is the sum of curves A and B and shows the total activation due to irradiation with the above defined flux spectrum while curve C shows the total activation resulting from irradiation with only thermal neutrons. Curve D shows the value of using a suitably balanced flux spectrum. As shown by curve A, a substantially pure epithermal flux would be most desirable from the standpoint of uniformity. Such a flux is, however, very expensive. Moreover, thulium-169 reacts to a minor extent with epithermal neutrons according to the reaction "'Im(n,2n)*Tm. The thulium-168 emits gamma radiation, which increases the shielding requirements. Accordingly, we usually prefer to use a flux approximating curve D. In some cases, uniformity may be sacrificed to obtain a lower gamma radiation and a flux containing less than this proportion of epithermal flux may be used.

The heat source of this invention may be used in any attitude without adverse affect on its performance. The fuel elements 11 may be repeatedly reirradiated because only about 10 percent to 20 percent of the 'lm O is converted during each irradiation. During each irradiation some '"Tm o is produced but it is a minor amount and has been discounted in the explanation of this invention.

The device described herein is only meant to be illustrative of the invention. The definition of the invention is found in the following claims:

1. A heating device comprising:

a housing having solid upper and lower heads connected by a hollow body, said lower head having therein a plurality of manifolds;

a plurality of coolant tubes in the body, said coolant tubes mating at both ends with said manifolds, a portion of each coolant tube being perpendicular to the upper and lower heads and a portion of each coolant tube being parallel to the upper and lower heads;

means for introducing a coolant into the coolant tubes;

a plurality of fuel tubes spaced from and arranged in clusters about each of the coolant tubes, said fuel tubes being perpendicular to the upper and lower heads and parallel to a portion of the coolant tubes;

fuel elements containing an elongated radioactive thulium oxide rod greater than 0.04 inches in diameter housed within the fuel tubes and removable therefrom;

a heat-exchange medium filling the space between the coolant tubes and the fuel tubes; and a backup cooling system separate from said coolant tubes for introducing coolant into the body at a predetermined time.

2. The device of claim 1 and further comprising a plurality of removable plugs in the upper head mating with the fuel ele ments and maintaining said fuel elements in the fuel tubesv 3. The device of claim 2 wherein the fuel elements comprise a gripping portion at the top thereof, an encapsulated fuel portion shorter than the distance between the upper and lower heads an means for projecting the gripping portion at least part way through the upper head upon removal of the mating plug therefrom.

4. The device of claim 3 wherein the fuel elements have a slotted connecting member of smaller diameter than the encapsulating material connected to the end of the encapsulating material closest to the upper head; a sheath of the same diameter as the encapsulating material positioned over said connecting member and slidable in relation thereto, said sheath having an internal roll pin positioned within the slot in the connecting member and slidable therewith; and a spring positioned about the connecting member between the encapsulating material and the sheath urging the sheath away from the encapsulating material, whereby removal of the mating plug from the upper head results in movement of the sheath and gripping portion through the upper head until the roll pin contacts one end of the slot preventing further movement of the sheath.

5. The device of claim 4 wherein the heat-exchange medium is compacted powdered metal and the thulium oxide rods are about 0.171 inches in diameter.

6. The device of claim 5 wherein the heat-exchange medium is tungsten and the encapsulating material is selected from the class consisting of titanium, beryllium and zirconium.

7. A fuel element comprising a cladding material encapsulating a rod greater than 0.04 inch in diameter containing a radioisotope of thulium oxide substantially uniformly distributed throughout the rod and further comprising a slotted connecting member of smaller diameter than the cladding material connected to one end of the cladding material; a sheath of the same diameter as the cladding material positioned over said connecting member and slidable in relation thereto, said sheath having an internal roll pin positioned within the slot in the connecting member and slidable therewith; and a spring positioned about the connecting member between the cladding material and the sheath luring the sheath away from the cladding material until the roll pin contacts one end of the slot preventing further movement of the sheath.

8, A method of utilizing thulium for heating comprising:

encapsulating and forming Tm O into elongated rods greater than 0.04 inches in diameter; irradiating the encapsulated rods in a neutron flux comprising at least onethird epithermal neutrons to produce Im Q, substantially evenly distributed throughout the rods;

maintaining the irradiated rods in heat exchange relation ship to a coolant;

and reirradiating the rods by introducing the encapsulated rods into a neutron flux comprising at least one-third epithermal neutrons.

9. The method of claim 8 wherein the rod is about 0.171 inches in diameter. 

2. The device of claim 1 and further comprising a plurality of removable plugs in the upper head mating with the fuel elements and maintaining said fuel elements in the fuel tubes.
 3. The device of claim 2 wherein the fuel elements comprise a gripping portion at the top thereof, an encapsulated fuel portion shorter than the distance between the upper and lower heads an means for projecting the gripping portion at least part way through the upper head upon removal of the mating plug therefrom.
 4. The device of claim 3 wherein the fuel elements have a slotted connecting member of smaller diameter than the encapsulating material connected to the end of the encapsulating material closest to the upper head; a sheath of the same diameter as the encapsulating material positioned over said connecting member and slidable in relation thereto, said sheath having an internal roll pin positioned within the slot in the connecting member and slidable therewith; and a spring positioned about the connecting member between the encapsulating material and the sheath urging the sheath away from the encapsulating material, whereby removal of the mating plug from the upper head results in movement of the sheath and gripping portion through the upper head until the roll pin contacts one end of the slot preventing further movement of the sheath.
 5. The device of claim 4 wherein the heat-exchange medium is compacted powdered metal and the thulium oxide rods are about 0.171 inches in diameter.
 6. The device of claim 5 wherein the heat-exchange medium is tungsten and the encapsulating material is selected from the class consisting of titanium, beryllium and zirconium.
 7. A fuel element comprising a cladding material encapsulating a rod greater than 0.04 inch in diameter containing a radioisotope of thulium oxide substantially uniformly distributed throughout the rod and further comprising a slotted connecting member of smaller diameter than the cladding material connected to one end of the cladding material; a sheath of the same diameter as the cladding material positioned over said connecting member and slidable in relation thereto, said sheath having an internal roll pin positioned within the slot in the connecting member and slidable therewith; and a spring positioned about the connecting member between the cladding material and the sheath luring the sheath away from the cladding material until the roll pin contacts one end of the slot preventing further movement of the sheath.
 8. A method of utilizing thulium for heating comprising: encapsulating and forming 169Tm2O3 into elongated rods greater than 0.04 inches in diameter; irradiating the encapsulated rods in a neutron flux comprising at least one-third epithermal neutrons to produce 170Tm2O3 substantially evenly distributed throughout the rods; maintaining the irradiated rods in heat exchange relationship to a coolant; and reirradiating the rods by introducing the encapsulated rods into a neutron flux comprising at least one-third epithermal neutrons.
 9. The method of claim 8 wherein the rod is about 0.171 inches in diameter. 